Implement Client Feature Flags and Sender Prefixmodes in Protocol and UI

[justjanne]

In short

• Transmit in the RegisterClient message the client features as int32
• Make the current SignalProxy available within of the thread for the
  current user
• Store the features of a peer in a flag in it
• Use Peer features to determine how to serialize or deserialize a
  message
• Add the prefixes to the UI when formatting the sender

Dependencies

• Due to the dependency on being able to query the source peer of a message, and the target peer, this PR depends on Let users see a list of their own connected clients, and disconnect them #302

Status

• Implement Protocol
• Implement UI
• Implement backwards-compatible (de-)serialization

Rationale

There have been multiple feature requests made asking to support sender modes, and displaying them in the client.

[Nevcairiel]

Can we keep the sender prefixes optional? I never missed them, and don't really want them.

[justjanne]

@Nevcairiel Sure, this can easily be made a UI setting